Both hard and soft contact lenses are typically manufactured by obtaining a contact lens blank, button or disk (hereinafter “blank”). The plastic lens blank is made by polymerizing a monomer mixture. The blank is machined into an optical shape providing a fixed correction (front curve and base curve) for the user's eye. The contact lens blank is shaped into the optical form by machining the blank. One common method is to use a cutting tool or lathe that removes sufficient material from either or both sides of the blank to give the plastic material the characteristic optical shape, a front curve and base curve, relating to its degree and strength of correction. In order to shape the blank, the material must be connected to a lathe or other machine and then cut. The machining must be done carefully, since the tolerances relating to the correction require careful control of the ultimate dimensions. With this in mind, the blank must be held in its position in a very stable and predictable location. The most common shaping method is to imbed the blank or button in a conventional “blocking wax” on a mandrel or other holder mechanism. The blocking compound holds the blank in a fixed unmoving position with respect to the cutting edge and can aid in attaining a desired shape. In this way, both the front curve and the base curve can be cut with great accuracy and the correction of the lens can be set using the typically computer controlled machining parameters. In typical contact lens manufacture, a mandrel is dipped into melt blocking wax and is then manually wiped free of excess material. The contact lens is then imbedded into the adhesive and then mounted on a jig on the shaping machine.
Conventional blocking waxes for holding a contact lens blank in position on a lathe have been typically aqueous insoluble organic thermoplastic materials. These materials require non-aqueous solvent removal and cleaning once the lathing, machining or other shaping steps are completed. In such a cleaning method, the contact lens and at least some of its blocking compound, along with the mandrel, in certain instances, are immersed in organic solvent such as 1,1,1-trichloroethylene, chloroform, cyclohexanone or other such organic solvent material. While the solvent adequately removes the material from the contact lens and mandrel, the resulting solution of blocking wax in solvent is hazardous waste and employee contact with the solvent can be irritating or disagreeable to the user.
Somewhat more recently, water-soluble blocking “waxes” have been proposed. Bonafini, Jr. et al., U.S. Patent Publication No. 2003/0119944 A1 teach a blocking wax composition comprising a variety of water soluble polymer materials and from about 10 to 50 wt % of a discontinuous solid phase. The solid phase suggested by Bonafini, Jr. et al. includes aluminum oxide (Al2O3), calcium oxide (CaCO3), titanium oxide (TiO2), silicon oxide (SiO2) and similar aqueous insoluble materials. The materials disclosed by Bonafini, Jr. et al., while water soluble, are based on polyethylene glycol polymers. Such materials have super cooling characteristics and do not readily solidify at low temperatures passing through a super cooled liquid phase. This characteristic of PEG based materials make them unsuitable for contact lens manufacture due to the delay in solidification and the uncertainty in maintaining the contact lens in the appropriate position.
The material suggested by Example IV in Bunnelle et al. is a material having a melting point substantially greater than 170° F. Such high melting materials are difficult to be used by the operating personnel, since materials of that temperature are not easily dealt with when used in the manual techniques common in today's contact lens manufacturing procedure. Lower temperature melt materials are required for comfort and safety of operating technicians. Neither Bunnelle et al. or Bonafini, Jr. et al. suggest the use of a material that will rapidly solidify at relatively low temperatures and that will remain substantially liquid at modest elevated temperatures in the range of about 135–170° F. promoting rapid and efficient mounting of contact lens in the blocking material and machining the material into a useful lens.
Bunnelle et al., U.S. Pat. No. 5,459,184, teach a moisture actuated hot melt adhesive. In Bunnelle et al., at Column 9, Example IV, is a contact lens blocking or mounting composition comprising a base polymer, hydrophilic additives and a filler such as calcium carbonate. On the whole, Bunnelle et al. suggest a specific polyalkyleneimine polymer, additive materials that have some hydrophilicity combined with a plasticizer and an aqueous insoluble filler. Fillers discussed in the Bunnelle et al. application include calcium carbonate (CaCO3), zinc oxide (ZnO), alumina (Al2O3, clay, titanium dioxide (TiO2), talc, carbon black (C0) and other similar substantially aqueous insoluble filler materials.
Neither Bunnelle et al. nor Bonafini, Jr. et al. suggest the use of water soluble filler materials. The blocking materials suggested by Bunnelle et al. and Bonafini, Jr. et al. have some utility in contact lens manufacture, but appear to be slow in dissolving in warm water and tend to require substantial time for a fully cleaned contact lens to be obtained after immersion in either warm water or aqueous cleaning solutions. In particular, we have observed that, upon immersion of blocking compounds such as that shown in Bunnelle et al., the material slowly dissolves but remains mechanically intact even as the final materials are removed by the cleaning solution.
A substantial need exists for a blocking compound that can be used by contact lens technicians without discomfort caused by excess melt temperatures and using materials that rapidly solidify to maintain the contact lens in the right position. Further, the contact lens blocking compound can be easily removed by the action of water or other aqueous cleaning solutions in a rapid process. The ideal materials will, upon contact with water, absorb water, become mechanically compromised and quickly break apart and become rapidly removed from the surface of the lens materials.